How Rod Lengths and Ratios Affect Performance
Changing the length of the rods with respect to the stroke of the crankshaft offers some advantages in certain situations.
The relationship between bore and stroke impacts the RPM range where an engine develops peak torque and horsepower.
Performance engine builders are always looking at changes they can make that will give their engine an edge over the competition. Rod ratio is one of those factors that may make a difference. Changing the length of the rods with respect to the stroke of the crankshaft offers some advantages in certain situations, and may allow the same number of cubic inches to deliver a little more power or a little longer ring life (take your pick). But experts disagree as to whether or not changing rod ratios really makes that much difference.
Rod ratio is the mathematical relationship between the overall length of the connecting rods and the stroke of the crankshaft. Divide rod length by the crank stroke and you get the rod ratio. For example, say you’re building a stock small block 350 Chevy with 5.7-inch rods and a 3.48 inch stroke. The rod ratio in this engine would be 5.7 (rod length) divided by 3.48 (stroke), which equals 1.64.
If you build the same 350 engine with longer 6-inch rods, the rod ratio becomes 1.72. And if you are building a 383 stroker with 6-inch rods, the rod ratio becomes 1.6 due to the longer stroke (3.750 inches).
What do these numbers mean? They express a geometric relationship between the rods, crankshaft and pistons. The lower the rod ratio, the greater the side forces exerted by the pistons against the cylinder walls. This increases wear on the piston skirts and cylinder walls, and creates a higher level of vibration inside the engine. The increase in friction can also elevate coolant and oil temperatures.
On the other hand, lower rod ratios do have some advantages. Shorter rods mean the overall height of the block can be shorter, which means the overall weight of the block can be lighter. The engine will typically pull more vacuum at low RPM, which means better throttle response and low end torque (good for street performance and everyday driving). Spark timing can be advanced a few degrees for some additional low speed torque, and the engine is less prone to detonation, which can be a plus in turbocharged, supercharged or nitrous applications.
Connecting rods come in various styles and lengths. Choosing the one that’s “right” for a given application depends more on strength, loading and RPM than rod ratio.
What about longer rod ratios? Using longer connecting rods with the same stroke reduces the side loading on the pistons, which reduces friction. It also increases the piston dwell time at Top Dead Center. Holding compression for maybe half a degree of crankshaft rotation longer at TDC improves combustion efficiency and squeezes a little more power out of the air / fuel mixture. Typically, an engine with a higher rod ratio will produce a little more power from mid-range to peak RPM.
Longer rods require the wrist pin to be located higher in the piston, or the engine has to have a taller deck height to accommodate longer rods. Longer rods also mean shorter and lighter pistons can be used, so the additional weight of the rods is more or less offset by the reduced weight of the pistons.
One of the disadvantages of longer rods and a higher rod ratio is that low RPM intake vacuum is reduced somewhat. Reduced air velocity into the engine hurts low speed throttle response and torque, which is not good for everyday driving or street performance, but works well on a high-revving race engine.
Some engine builders say a “good” rod ratio is anything 1.55 or higher. Production engines may have rod ratios that range from 1.4 to over 2.0, with many falling in the 1.6 to 1.8 range. Four cylinders tend to have lower rod ratios (1.5 to 1.7 range) while many V6s have somewhat higher rod ratios of 1.7 to 1.8. As for V8s, they typically range from 1.7 to 1.9. Often, the rod ratio is dictated by the design and deck height of the block, and the pistons, rods and crank that are available to fit the block.